A gull which doesn’t quite fit the bill

A closer look at the Ravensthorpe ‘Caspian’ Gull

Generally regarded as a Caspian Gull, this second-calendar year (first-summer) bird has been visiting Ravensthorpe Reservoir since early August. From the initial images obtained by Gary Pullan, it looked marvelously ‘snouty’ and long-legged – two features widely associated with Caspian Gull.

Second calendar year gull, Ravensthorpe Res, 16th August 2019 (Gary Pullan)

There were, however, some characteristics which simply did not ring right for Caspian Gull, leading Gary and I to debate its identity, which swung from Caspian to Yellow-legged, though Caspian x Yellow-legged hybrid and then back to Caspian. John Moon chipped in with a better image, which did not really change anything at the time.

Second calendar year gull, Ravensthorpe Res, 19th August 2019 (John Moon)

On 31st August, I managed to get some digiscoped shots (below) of which some, after scrutiny, were perhaps more suggestive of Yellow-legged Gull – not least because of the bill structure.

Summary of features based primarily on images taken on 31st August.

  • Large and lanky and legs long and good for Caspian
  • Shortish (closed) wings but renewed primaries probably still growing
  • Mantle shade of grey too light for Yellow-leggedGul but OK for Caspian (but see comments)
  • Bill long but heavy, with large gonys – looks in some images to be fine for Yellow-legged Gull but too chunky for Caspian but in others ok(ish) for Caspian
  • Underwing coverts quite dark – darker than I would expect for Caspian but Malling Olsen states some Caspians can have quite dark underwings (and see comments below)
  • Head shape, long forehead and eye position ok for Caspian and looks ‘snouty’

I forwarded a set of images to Carl Baggott – the Leicestershire Recorder, ‘King of Shawell’ and a man with a true passion for gulls and with extensive experience of Caspian and Yellow-legged Gulls in eastern Europe and Yellow-legged in Portugal.

Carl kindly commented as follows:

This is quite a difficult bird, but I don’t get a Caspian Gull feel from the images. It seems closer to YLG and I am not too concerned about the lightness of the grey feathers as this is quite variable at that age. The tertials favour YLG also, as does the large head and bill. The snouty description of Caspian Gull head is overstressed and doesn’t really help with most gulls. As you say some Caspian Gulls can have dark underwings and I have photographed birds in Germany with similar underwings. Most 2CY Caspian Gulls have pale inner primaries or a venetian blind effect across the inner primaries.

I have seen birds like this at Shawell and left them unidentified. It can be very difficult with ones like this as you don’t know their origin. I have seen Yellow-legged Gulls in Portugal that are easily confused with Caspian Gulls and even American Herring Gulls.

On the question of the possibility of a hybrid:

It could well be [a hybrid], but difficult to say for certain. There is nothing to really hang your hat on. A colour-ring is always useful as you know so at least you know where it’s from. Hybrids are easier as adults or first-winters usually as you can look at coverts (Caspian Gulls especially) on the young birds and primaries on adults. Then it comes down to matter of opinion unless you know the species of the parents. Hybrid is always the go to, but there is a great deal of variation in pure birds.

The above not only highlights the difficulty of identifying ‘odd’ gulls in the field but also the problems with trying to identify them from images taken from different angles and in different poses – for example, see the apparent change in head shape and bill thickness in the above images.

For a set of images of similarly-aged Yellow-legged and Caspian Gulls in The Netherlands  see here.

Comments welcomed!

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